Project Summary: Radiotherapy (RT) can result in progressive neurodegeneration and cognitive impairment in elderly patients. The cause for increased risk of neurodegeneration in the elderly after RT may involve age- related changes in pathophysiological mechanisms such as irradiation-induced neuronal cell death and neuronal senescence/neurodegeneration. Recent studies demonstrated heterogeneity of neuronal population in the brain. We hypothesize that distinct subsets of neurons will be differentially affected by ionizing radiation (IR). Aging may alter the balance among such neuronal subtypes and their responses to injury. One of the common aging and IR-related changes in cell populations is senescence when cells gradually degenerate and lose their functional characteristics. microRNA (miR) -711 is upregulated in the hippocampus in a mouse AD model, in neuronal DNA-damage models, and after brain injury. We have demonstrated that miR-711 promotes IR/DNA damage-induced neuronal cell death by down-regulating expression of multiple pro-survival genes and induces neuronal senescence through up-regulation of molecules such as p21. miR-711 also inhibits DNA repair mechanisms. The objective of this proposal is to determine the differences in neuronal subtypes and their responses to IR and examine the potential neuroprotective properties of a miR-711 inhibitor in both young-adult and aged mice. Our central hypothesis is that miR-711 inhibition can reduce IR-induced neurodegeneration and limit long-term cognitive dysfunction; the induction of miR-711 and benefits of miR-711 inhibition are significantly increased in the aged mice due to differences in neuronal subtype susceptibility to IR-induced DNA damage. Here, we will use single massively parallel single-nucleus RNA-seq, neurobehavioral, histological and flow cytometry approaches to test our novel hypotheses as outlined in following specific aims: Aim 1: Determine the differences in mechanisms driving the IR-induced neurodegeneration in young-adult and aged animals. We hypothesize that brain irradiation will cause more severe neurodegeneration in aged animals due to age-related changes in subtypes of neurons and their susceptibility to DNA damage. We will utilize snRNAseq to assess the effects of aging and IR on transcriptomic changes promoting neurodegeneration in sub-types of neuronal populations at one week and three months post-irradiation; IR-induced cognitive deficits will be examined behavioral assessments. Aim 2: Determine the effect of miR-711 inhibitor in the irradiated brain of young animals and aged animals. We hypothesize that inhibition of miR-711 after IR will attenuate neuronal cell death and senescence progression and improve cognitive deficits in young adult mice. The effects of miR-711 inhibitor may be particularly beneficial in aged animals due to age-related changes in neuronal subtypes that increase neurodegenerative and senescence-like responses.